One type of linear actuator includes a threaded rod that passes through a threaded hole in the rotor of an electric motor, so that rotation of the rotor advances the rod. U.S. Pat. No. 2,446,393 shows such an actuator. Where the rod must apply very large forces, such a direct motor-to-rod coupling has the disadvantage that large thrust forces are transmitted through the motor rotor, so that conventional low load precision motor bearings cannot be utilized. Such high loads are also difficult to carry through conventional threaded bearings. Direct sliding metal-to-metal contact of the threaded members generates unacceptable friction and wear when lubricants are merely applied to the surfaces. Threaded ball bearing mechanisms cannot withstand very high loads without suffering fatigue damage, and conventional threaded hydrostatic bearings are also found incapable of supporting very high loads without an external centering bearing.